Porcine reproductive and respiratory syndrome virus (PRRSV) was discovered in the late 1980s as the cause of severe reproductive failure in sows and gilts and is one of the most important pathogens in the swine industry. Infection of sows and gilts can lead to late term abortion, early farrowing and the birth of weak-born piglets, while infected boars show decreased sperm quality and virus excretion in the semen.
In addition, PRRSV is also found to be involved in the porcine respiratory disease complex in young pigs, causing respiratory problems in combination with secondary viral and bacterial infections. The virus shows a restricted in vivo cell tropism with alveolar macrophages being the main target cell.
PRRSV is an enveloped positive single-stranded RNA virus of the family Arteriviridae and order Nidovirales (1) with approximately 15 kb in length, consisting of 9 open reading frames (ORFs). The virion consists of a nucleocapsid core that is built up by nucleocapsid protein (encoded by open reading frame 7, ORF7) in association with the viral RNA. The nucleocapsid is surrounded by a lipid envelope in which six structural proteins are embedded: the glycoproteins GP2 (ORF2a), GP3 (ORF3), GP4 (ORF4) and GP5 (ORF5), and the non-glycosylated proteins M (ORF6) and E (ORF2b). GP5 and M are considered to be the most abundant proteins in the envelope, while the other envelope proteins are present in lower amounts. The ORF1a and ORF1b situated at the 5′ end of the genome encode non-structural proteins.
Similar to many other RNA viruses, PRRSV shows a large genetic variability, which is reflected in variation in virulence, interaction with the immune system and antigenic properties of viral proteins. Virus strains are usually classified within a European (EU) and a North-American (NA) genotype, based on ORF5 and/or ORF7 sequences, although a high degree of variability exists within genotypes.
PRRSV has acquired a number of properties that allow escape from the host's protective immunity. These properties are late production of virus-specific antibodies after one or two weeks upon infection; with such antibodies being unable to reduce in vitro virus replication in primary porcine alveolar macrophages (PAM); and with the much needed virus-neutralizing antibodies appearing at low levels around three to four weeks after infection thus too late to influence the acute phase of viremia (1, 2).
Despite this weak virus-neutralizing antibody response, the presence of sufficient amounts of such virus-neutralizing antibodies at the onset of infection can offer protection against virus replication in the lungs, viremia and transplacental spread of the virus, indicating that PRRSV-specific neutralizing antibodies can contribute in part to protective immunity (2,3).
The PRRS viremia was found in the blood of infected pigs with neutralizing antibodies, indicating the humoral immune response alone did not confer solid protection. The cell-mediated immunity (CMI) has been shown to play an important role in clearing PRRSV (4). The development of the CMI response in infected pigs, as determined by lymphocyte blastogenesis and adaptive cytokine production (e.g. Interferon gamma; IFN-gamma) was found delayed and became detectable in the in vitro recall response of peripheral blood mononuclear cells (PBMCs) around 4-8 weeks post infection, which correlated with the development of neutralizing antibodies (5-7). The IFN-gamma plays a key role in cell-mediated immune responses against a variety of cytopathic viral infections in animals. In PRRSV-infected pigs, the IFN-gamma mRNA was detected in the lymph nodes, lungs and peripheral blood mononuclear cells (7).
The search for antigenic regions across the entire PRRSV structural proteome representing virus-neutralizing antibody inducing B cell epitopes and IFN-gamma eliciting T cell epitopes has been one of the most challenging topics in veterinary viral immunology over the past two decades. Representative articles showing such epitope mapping outcome as a result of the cumulative efforts by the global PRRSV research community are herein provided as references (8-10).
Despite the commercial availability of modified-live vaccines (MLV) as well as killed PRRSV viral lysate vaccines, the control of PRRSV related diseases still remains problematic. One major problem is efficacy in that PRRSV vaccines are efficacious against homologous, but not heterologous, challenge. In addition, safety issues for the MLV have been reported in the field. Modified live vaccines are not suitable for use in pregnant sows, gilts and in boars as vaccination may result in shedding of vaccinal virus in semen. Modified live virus vaccines can persist in vaccinated animals. Transmission to non-vaccinated animals and subsequent vaccine-virus-induced disease have been reported. Furthermore, there is an urgent need for the development of a marker vaccine to allow differentiation between infected and vaccinated pigs, thus facilitating traceability and control of PRRSV infection.
In summary, there remains an urgent need to design immunogenic PRRSV peptides comprising distinct functional B and T cell epitopes, that are capable of inducing protective antibodies and cellular immune responses, as well as vaccine formulations incorporating these designer peptides to allow for cross-protection of PRRSV strains in swine. With the availability of these rationally designed and molecularly characterized immunogenic peptides, there is also the need to identify antigenic peptides capable of being recognized by antibodies from infected pigs, and to use these designer peptides to develop a set of diagnostic tests, thus a diagnostic system, for serological identification of infected versus vaccinated animals to allow for effective control of PRRSV infection. Finally, there is this need to develop means for low cost manufacture and quality control of such peptide-based marker vaccine and diagnostic system for wide application to effectively monitor and control the PRRS disease.
References
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